1. Field of the Invention
The present invention relates to a form measuring instrument such as a roundness measuring machine which measures a displacement in synchronization with an angular position of rotation of a workpiece that is rotating about a specified rotation axis, and a calibration method and calibration program therefor.
2. Description of the Related Art
Conventionally, there has been known a roundness measuring machine which receives a workpiece on its table, rotates the table, and makes a contact tip (e.g., a contact piece) scan the surface of the workpiece (see JPH5-231864A, JP2551698B). Such a roundness measuring machine rotates the table to detect a displacement of the contact tip in an X-axis direction and in a Z-axis direction. That is, the roundness measuring machine performs the measurement while locking the contact tip in a Y-axis direction. Therefore, in order to obtain a highly accurate measurement, it is important to calibrate the position of the contact tip in the Y-axis direction.
Normally, calibration of the position of the contact tip in the Y-axis direction is performed in the following manner. First, a reference sphere is positioned at the center of rotation of the table, and a Y-axis direction adjustment screw of a detector holder is manually twisted while the contact tip is fixed in contact with the reference sphere. Then, the adjustment screw is stopped at a position where X-axis and Z-axis direction peaks of a level meter (X-axis and Z-axis direction peaks of the sphere) are observed.
However, the above calibration method relies only upon human perceptions and tends to generate an error. The Y-axis direction error causes errors in the angular position and X-axis position of the workpiece and in the measured values, and such errors are greater at a position more proximal to the center of rotation of the table.
For example, when there is a Y-axis direction error, a measurement of the flatness of an optical flat (having a cylindrical shape) which is performed by inclining the optical flat will result in that different inclination values will be detected at the center and at a position more distal, and that the shape of the top surface of the optical flat, which should be a flat shape by right, will be measured as a warped shape. Further, in a measurement of roundness or cylindricity, the smaller the diameter of the workpiece, at the farther position from the 0-degree position of the workpiece, the contact tip will gain contact with the workpiece. Thereby an error may be caused in an analysis calculation result, and the centering may not be converged.